Norfolk Amateur Radio Club

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Merry Tucker
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1 Norfolk Amateur Radio Club The Transmitter & Transmitter Interference Nick M0HGU & Steve G3PND

2 Plan for the Day The Transmitter Introduction, Block diagrams Oscillators, Buffers & Multipliers Modulation Mixing (up), PAs, ALC & SWR Transmitter Interference Stability & Drift Unwanted emissions Operator faults Harmonics RF or AF?

3 The Transmitter We are revising & extending the intermediate material What does a transmitter need to do? Create an RF signal inside the amateur band For the signal to be nice and clean For the signal to be stable For the signal to have minimum band width for the type of transmission For the signal to have the correct power output For the transmitter to have minimum output on other frequencies

4 The block diagram An SSB transmitter using mixers to generate the final RF (though it is common now for the modulator to be switchable to generate AM, SSB or FM)

5 The block diagram A FM transmitter using frequency multipliers to generate the final RF

6 Oscillators Steve G3PND

7 Modulation We need to look at the following: AM SSB FM Digital

8 Modulation types You will remember from intermediate (and foundation) the two basic types of modulation:

9 AM Amplitude Modulation: Generated by mixing the Audio Frequency & either Radio Frequency or Intermediate Frequency in an un-balanced

10 AM The unbalanced mixer produces all 4 mixing products at it s output

11 AM or single balanced mixer

12 AM The single balanced mixer produces Only 3 of the possible outputs and is configured to include the carrier (usually)

13 AM The Modulation Depth indicates how modulated the carrier is, more modulation results is a louder received audio. When m=1 the carrier is doubled on the peaks and 0 in the troughs M>1 = distortion Aim for m=0.8 m=0.9

14 AM When m=1 the amplitude of each sideband is exactly half the carrier amplitude.

15 SSB Here we don t want the carrier nor one of the side bands (USB or LSB) and a double balanced mixer is used.

16 SSB The unwanted sideband is removed by a filter, the carrier is suppressed by the DBM. We do not need both sidebands as they contain the same information So saving bandwidth, & power, one sideband cannot exceed ¼ the power of the carrier (see AM)

17 SSB/AM If a DC offset is applied to the RF input of the DBM then the carrier will not be rejected and an AM signal will result.

18 FM Remember the amplitude of the FM signal is constant and the frequency changes with the amplitude of the audio input. This can be done in two ways Changing the frequency of the oscillator Changing the phase of the oscillator output

19 FM A colpitts oscillator and be pushed & pulled

20 FM By applying the audio to the buffer after the oscillator it is possible arrange to change the instantaneous phase of the signal, this has the same effect as changing it s frequency. (not to be confused with Phase shift keying see later)

22 FM Peak deviation: Is the maximum amount that the frequency may vary - there is no theoretical limit but FM entertainment is limited to 75KHz Amateur are either 5KHz or 2.5KHz Deviation Ratio:

23 FM Modulation Index Is the ratio of Peak Deviation to Max Audio Frequency e.g. VHF FM Broadcasts 75KHz/15KHz = 5 This is Wide Band FM MI>1 e.g. Amateur 2M 2.5KHz/3KHz = 0.8 This is Narrow Band FM MI<1

24 FM Bandwidth Mathematically it can be argued to be infinite

25 FM The sidebands extend beyond the Peak Deviation, increasing the PD just means more of the sidebands have significant power, it will not alter the separation of the bands (if a single tone is used). Real audio is a range of tones and the spectrum will be continuous A general rule is Carson s Rule: Bandwidth = 2 * (Max Audio Freq. + Peak Deviation)

26 FM Carson s Rule: e.g Amateur 70cm transmission 2*(3KHz+5KHz) = 16KHz hence 25KHz channel separation At VHF 2*(2.8KHz+2.5KHz) = KHz channels are used

27 CW Carrier Wave or Morse Code Can be thought of as AM with a modulation depth of 10% There will be sidebands, these depend on speed and shape of the keying A fast rise time will cause sidebands several KHz wide think of the square wave keying as having many high frequency components modulating the AM signal

28 CW A rise time of around 5ms is considered a good compromise between readability and suppression of key clicks. A small R/C filter can be used to achieve this, or much more sophisticated methods can be devised.

29 Key Clicks Key clicks evident Clean signal

30 Data Early data was RTTY, using a 5bit code, followed by PSK31, MFSK16, Packet etc. There are three ways to Tx data Data is converted to audio tones which are sent to a FM transmitter Data is converted to audio tones which are sent to an SSB transmitter Directly modulating the carrier

31 Data On FM, the carrier is modulated by one or more audio tones This is Audio Frequency Shift Keying, be aware that there will be a carrier even when no data (audio sub carrier) is present and the bandwidth is determined by the highest audio tone and the peak deviation

32 Data AFSK spectrum (7.002MHz carrier RTTY & KHz)

33 Data If we feed the data directly into the FM modulator (varicap diode in a modified colpitts oscillator) here we do not convert to audio and the bandwidth is small. This is FSK (7.002MHz carrier RTTY & KHz)

34 Data If we feed the tones used to modulate our FM signal above into an SSB transmitter the result is identical to that of FSK (7.002MHz carrier RTTY & KHz)

35 PSK Not in the syllabus but Binary Phase Shift Keying (at 31 baud)

36 Emission codes AFSK on FM is F2B F = FM 2 = Audio sub carrier B = Data for automatic reception FSK is F1B 1 = digital signals with no sub carrier SSB AFSK is J2B J= SSB

37 Mixing up, Pas, ALC & SWR Steve G3PND

38 Transmitter Interference Stability & drift You will remember from earlier that we need our Tx to: Create an RF signal inside the amateur band For the signal to be nice and clean For the signal to be stable

39 Stability & Drift Drift is where the (or one of the) master oscillators changes frequency over a period of time. There are several possible causes: Temperature change Supply voltage change Change to output loading / stray capacitances Mechanical shock Ageing

40 Temperature The temperature can affect inductance and capacitance in tuned circuits all transmitters warm up usually 30 minutes or so is enough Long overs (especially high duty cycle ones like data) can have an effect. It is possible to mix +ve & -ve temperature coefficient components to get close 0 temperature drift

41 Temperature

42 Supply Voltage/Chirp Supply voltage can affect the capacitance of active devices Use a separate, isolated, regulated supply for master oscillators Keep oscillators & buffers powered continuously

43 Output Loading A change in loading can pull an oscillator, a buffer amp will help as it isolated the oscillator from the load Stray capacitance, a hand, can also pull the frequency of an oscillator, screening with a tin can will avoid this (and spillage into other circuits) along with insulated couplings on controls

44 Mechanical shock Loose free wound inductors will move if knocked and so change their value. Always use stout connections, if te coil is on;y 2-3 turns and made with thick wire they should be OK. Capacitors should be high quality and again robustly connected

45 Ageing This is more an effect of crystals. The specification is given by the manufacturer in ppm (parts per million) e.g. a crystal with 5ppm initial accuracy and 10ppm/yr could be up to 15ppm off by the end of the first year so if 10MHz, it may be 150Hz off frequency

46 Stability & Drift Keep oscillators away from sources of heat the PA Ensure voltage supplies to oscillators are stable & well decoupled Screen sensitive sections to prevent stray capacitance effects and leakage of signals into or our of the circuit Buffer oscillators to reduce loading effects Use robust construction to reduce mechanical effects Allow for ageing od crystals by providing trimmer capacitors Use high quality components & construction methods

47 Unwanted Emissions etc. Steve G3PND

Technician License Course Chapter 2. Lesson Plan Module 3 Modulation and Bandwidth

Technician License Course Chapter 2 Lesson Plan Module 3 Modulation and Bandwidth The Basic Radio Station What Happens During Radio Communication? Transmitting (sending a signal): Information (voice, data,